1. Field of the Invention
The present invention relates to a signal detection method and apparatus for detecting a charge signal outputted from a radiation image detector, which is configured to receive radiation carrying a radiation image to record the image therein, as an image signal representing the radiation image by integrating the charge signal outputted from the detector with an integral amplifier and passing the integrated electrical signal through a low pass filter.
2. Description of the Related Art
Various types of radiation image detectors configured to receive radiation transmitted through a subject to record the radiation image of the subject and to output a charge signal in accordance with the image recorded therein have been proposed and put into practical use in the medical and other industrial fields.
One such detector uses a semiconductor material that will generate charges when exposed to radiation. Another type of such detector uses a storage phosphor sheet made of storage phosphor that will store radiation energy when exposed to radiation and emit stimulated luminescence when irradiated by reading light.
As for examples of the radiation image detectors using a semiconductor material described above, a so-called optical reading system that will read out the charge signal through the scanning of linear reading light, and a so-called TFT system that will read out the charge signal through on-off switching of TFT switches arranged two dimensionally have been proposed. In one of the radiation image detectors that use the storage phosphor sheet, the charge signal will be read out by receiving stimulated luminescence emitted through the irradiation of the reading light from the storage phosphor sheet with a linear sensor having photoelectric conversion elements arranged lineally, and converting the stimulated luminescence received by the sensor to an electrical signal through photoelectric conversion at the linear sensor.
Then, as described, for example, in the non-patent document entitled “Large-Area, Low-Noise Amorphous Silicon Imaging System” (R. B. Apte et al., SPIE Vol. 3301, p. 2–p. 8, 1998), the charge signal outputted from the radiation image detector is integrated by the integral amplifier (also referred to as “charge amplifier” as it amplifies charges) connected at the latter stage and detected as an image signal representing the radiation image. The wide use of the integral amplifier is due to the fact that it is easier to obtain a high sensitivity with ease of integration in manufacturing compared with I–V amplifiers and the like because of its circuit configuration.
Here, the amount of noise QN of the integral amplifier described above is express by the following formula:
      Q    N    =            Q      o        +                  C                  i          ⁢                                          ⁢          n                    ⁢                                    kT                          g              m                                ⁢                      (                                          f                N                            -                              1.8                                  t                  s                                                      )                              
where:
k: Boltzmann constant
T: absolute temperature
QO: fixed noise component
tS: sampling time
fN: noise bandwidth, for 1—pole LPF,
      f    N    =      1          4      ⁢                          ⁢      τ      τ : time constant
Cin: total input capacitance
gm: transconductance of integral amplifier
Thus, in order to reduce the noise, fN needs to be minimized, that is, the time constant τ of the low pass filter connected at the latter stage of the integral amplifier needs to be increased. In particular, the radiation image detectors of the optical reading system and those that use the storage phosphor sheet described above are slow in responding to reading light, requiring a longer sampling time ts, whereby fN needs to be further reduced.
However, the increase in the time constant τ of the low pass filter connected at the latter stage of the integral amplifier results in a longer transient response time of the low pass filter when integral amplifier is reset to discharge the charges integrated therein. Thus, if the signal reading needs to be completed within a predetermined sequence time, a less time may be allowed for the irradiation of reading light and the charge signal read out from the radiation image detector becomes inevitably small, resulting in degradation of the signal-to-noise ratio. On the other hand, if a predetermined signal-to-noise ratio is to be maintained, a certain time period needs to be secured as the irradiation time of the reading light. This requires a longer sequence time, thus resulting in a longer reading time for reading out the radiation image.